Since the advent of electrical circuits, systems, and electronic devices various methods have been conceived to test these devices in order to recognize flaws in the electronic circuits and to improve design. Common testing methods may include using a separate tool or meter which a user may utilize to make electrical contact with a circuit in order to measure various properties such as voltage (V), current (I), resistance (R), capacitance (C), inductance (L) or impedance (Z). Such measurements may be useful in many circumstances, e.g. in circuit design, for interrogating sensors which provide data in terms of these measurements, etc.
More recently, time domain reflectometry (TDR) has been used to provide a single-point measurement (transmit/receive at same point) for the reflection from the circuit. With TDR, a step or pulse is transmitted into a system, reflects off impedance changes in the system and returns to the original test point, where it is received. This provides the step or pulse response of an electrical system as a function of time. This has been used to measure the properties of a transmission line in order to, for example, find a short circuit. TDR is considered a ‘time domain’ measurement. It can be converted to the ‘frequency domain’ using a Fourier transform.
Vector network analyzers (VNA)/network analyzers (NA) measure the frequency domain response (transfer function) of a system. They transmit a series of sine waves of different frequencies into the electrical system, and receive the returning sine waves at the same point (reflection, called S11) and often other points as well (transmission, called S-parameters S12, S13, etc. . . . Sij). Most VNAs are two port (meaning they can transmit and receive from/to port 1 or port 2 only). Additional data acquisition control circuitry can be added to make them multi-port devices (more than two ports). However, multi-port VNA devices are prone to noise problems. Because of this, the multiple ports are generally utilized sequentially (i.e. to take one measurement with one port, and then another with the next port, progressing through all of the ports and port combinations sequentially) when conducting circuit parameter measurements. The frequency domain response provided by the VNA can be converted to the time domain response (TDR equivalent) using an inverse Fourier transform.
It is notable that TDR and VNA methods have significant disadvantages associated with their use when testing circuits. For example, these systems are not able to be utilized in a live setting. Because of this, in order to run tests using these tools, one must disconnect the circuit under test from its functioning environment. In many applications, such as power distribution networks that cannot be readily shut down, such a necessity is problematic. Additionally, as testing of a circuit occurs outside of its functioning environment, it is more difficult to find intermittent problems in a circuit, or problems that occur only under load (such as battery degradation). TDR and VNA methods are also expensive due to complications which arise when handling tests at higher bandwidths. For example, TDR requires fast sampling to acquire reflected signals. As such, increasing bandwidth requires more sophisticated internal processing circuitry which can handle the faster speeds. VNA methods require a precise sine wave generator and receiver, which at higher frequencies are more difficult to implement. Furthermore, the noise immunity from and interference with a circuit under test are problematic for TDR and VNA measurements.
Spectral time domain reflectometry (STDR) and spread spectrum time domain reflectometry (S/SSTDR) are two spread spectrum methods that have been applied in more limited circumstances, such as in testing signal paths for locating faults on live aircraft cables, power cables for undersea oil drilling platforms, and the like. However, such methods have not been utilized and/or more fully developed for more in-depth testing of circuit components, e.g. testing R, L, C, and Z in a circuit or system, for utilizing multi-port testing, and the like.